Tantalum is one of the most corrosion-resistant metals known; in most environments, it is almost as inert as gold or platinum. Tantalum's corrosion-resistance can be attributed to the presence of a thin layer of tantalum oxide (Ta.sub.2 O.sub.5) on the metal surface. In general, tantalum presents a non-wetting surface to most liquids. Tantalum parts used in corrosive environments generally exhibit extended longevity because thermal cycling does not affect its surface finish and corrosion by many conventional liquid metals and salts is minimal. In addition, tantalum can be cold worked and machined.
The majority of tantalum used in this country is by industries employing corrosive chemicals. About one half the tantalum used in the chemical industry is used to process sulfuric acid. Plants use tantalum heating surfaces to reconcentrate sulfuric acid generated from metal pickling, oil refinery operations, and petrochemical processing in the production of alcohols and ketones. Tantalum is also used in the production and handling of hydrochloric acid, hydrobromic acid, and derivatives thereof. Operations involving chlorine or chloride derivatives employ about one-fourth the tantalum used in the chemical industry. Plants employing aqua regia to refine precious metals use tantalum extensively since tantalum is also one of the few metals resistant to aqua regia. Tantalum is widely used in the production of high purity nitric acid and terephthalic acid. Industries that cannot tolerate impurities such as the pharmaceutical and food industries, use tantalum because of its negligible corrosion rate.
Pyrochemical and pyrometallurgical processes for nuclear materials employ tantalum components. Although highly corrosion-resistant, tantalum may not be completely inert in the extremely corrosive environments employed in the processing of various nuclear materials. Failed hardware can lead to the interruption or failure of a procedure which may result in added exposure to radioactive materials. Additionally, degraded or failed components must be discarded as radioactive waste.
Although tantalum crucibles have been used to contain corrosive liquids such as molten plutonium, various problems arise during processing. For example, in column 1, lines 40-53, of U.S. Pat. No. 5,383,981, to Jean A. H. de Pruneda entitled "Reusable Crucible For Containing Corrosive Liquids," which issued Jan. 24, 1995, it is stated that wetting, that is, adhesion, of the molten metal to the surface of the tantalum crucible causes " . . . chemical and mechanical corrosion of the crucible. The corrosive liquid adheres to the crucible surfaces, attacks the grain boundaries of the crucible material, penetrates along the grain boundaries, and eventually detaches grains of the crucible that can dissolve in and contaminate the liquid. The wetting of the crucible by the liquid metal also hinders the removal of the cooled product . . . ".
The corrosion-resistance of a tantalum component can be increased if the component is carburized. However, de Pruneda states in column 1, lines 54-63, that the resultant " . . . surface coatings do not remain bonded to the substrate, however, but are stressed during cooling of the melt. A cooled, solidified material like plutonium, for example, has a thermal expansion coefficient quite different from the container material, which causes the layers of tantalum carbide to fracture and rip off during the cooling and removal of the solid . . . ".
In contrast to the above-mentioned tantalum or surface-carburized tantalum components, the '981 patent teaches a non-wetting, corrosion resistant material comprising a tantalum or tantalum alloy substrate which is supersaturated with carbon. Example 1 of the '981 patent, column 4, line 51, through column 6, line 5, describes a procedure for making a carburized tantalum crucible. Assuming a parabolic growth rate and an Arrhenius temperature dependence, the mass of carbon needed to saturate the crucible with carbon is calculated. The thickness of TaC and Ta.sub.2 C layers having this mass of carbon is calculated. A thin-walled (0.287 cm) crucible is heated in methane at 1600.degree. C. for 3-4 hours until layers of TaC and Ta.sub.2 C of the chosen thicknesses are formed. The crucible is then "heat soaked" under a vacuum for 15-20 hours to allow the carbon present in the surface carbide layers of the crucible to diffuse into the crucible. It is important not to grow TaC and Ta.sub.2 C layers having thicknesses in excess of the calculated thickness because an undesirable surface tantalum carbide layer would remain after the heat soaking procedure.
The carbon-saturated tantalum material disclosed in the '981 patent is said to have non-wetting properties superior to either tantalum or surface carburized tantalum. Also, in column 4, lines 1-4, it is stated that alternative carbon sources such as solid carbon or acetylene gas can be used.
The process described in the '981 patent requires about 1 day of heating at 1700.degree. C. to carburize a thin-walled crucible. It may be desirable to reduce cost by shortening heating time. Although elimination of the heat soak step of the '981 patent would shorten processing time, the tantalum crucible, after carburizing and prior to heat soaking, has carbide coatings that must be removed before using it to contain corrosive liquids.
Therefore, an object of the present invention is to provide a process for carburizing objects of tantalum or tantalum alloy having increased corrosion resistance.
Another object of the invention is to provide a process for making reusable carburized objects of tantalum or tantalum alloy having a non-wetting surface.
A further object of the present invention is to provide a process for carburizing objects of tantalum or tantalum alloy using a non-flammable carbon source.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.